Piston type cryogenic apparatus



Nov. 15, 1966 R. DOLL ETAL 3,285,142

PISTON TYPE CRYOGENIC APPARATUS Filed April 1964 2 Sheets-Sheet 1[Mun/mes:

05657" DOLL Fen/v2 X. Ewe

Nov. 15, 1966 R. DOLL ETAL PISTON TYPE CRYOGENIC APPARATUS 2Sheets-Sheet 2 Filed April 1964 age United States Patent ice 3,285,142PISTON TYPE CRYOGENIC APPARATUS Robert Doll, Mettinghstrasse 1, andFranz Xaver Eder, Wadlerstrasse 21, both of Munich, Germany Filed Apr.6, 1964, Ser. No. 357,401 Claims priority, application Germany, Apr. 10,1963, D 41,347 18 Claims. (Cl. 91-401) This invention relates generallyto cryogenic apparatus and more particularly is concerned with suchapparatus using a piston reciprocating in a suitable cylinder, therebeing annular grooves provided on the bearing surfaces of the piston orcylinder and gas under high pressure flowing in the clearance spacebetween the piston and cylinder.

In cryogenic apparatus of the character with which this invention isconcerned, a working gas is expanded approximately adiabatically fromhigh to low pressure by producing mechanical work, the gas being therebycooled. Such apparatus are of two types, namely, expansion turbines andexpansion piston machines, both being used to produce low temperatures,particularly in the liquefying of gases which have low boiling points,and without pre-cooling through the use of other liquefied gases havinghigher boiling points. Examples of such gases which require cooling forliquefying are neon, hydrogen and helium. Since expansion turbines cangenerally effectively process large quantities of gas and low pressureratios, only expansion machines of the reciprocating piston type areused for medium to small liquefying and gas cooling installations.

In known piston type cryogenic machines the working piston is providedwith annular grooves on its surface and the clearance volume between thepiston and the cylinder surface is of right cylindrical configuration,that is, the juxtaposed surfaces are co-parallel with the theoreticaldistance between them parallel throughout. This clearance volume inprior devices has not been of equal dimension on opposite sides of thepiston, as will be seen. The clearance volume will hereinafter be calledthe space between the piston and the inner cylinder surface. In theprior structures, the annular grooves had two function, one-to improvethe pressure seal of the working chamber from the outer chamber,and,-twoto equalize the pressure around the circumference of the piston.These prior devices did not achieve a contactfree or floating stroke ofthe piston.

Because of the inability to achieve true floating stroke,

prior cryogenic apparatus of the piston type could be operated only atlow speeds, and even then wear on the bearing surfaces was unavoidable.

In addition to the above disadvantages, in prior devices introductionand expulsion of the gas was achieved through the use of mechanically orelectromagnetically operated valve mechanisms. These mechanismsrepresent additional structural parts, complicate the construction ofthe machine and impair its safety. In cases Where the working gas couldnot be permitted to come in contact with lubricating medium or in theproduction of very low temperatures where the use of oil lubrication isnot feasible for physical reasons, a fully floating piston stroke hasnot been possible in any prior apparatus of which we are aware.

The invention herein has as its most important objects the eliminationof the above disadvantages and the provision of a piston type cryogenicapparatus in which the piston reciprocates in a fully floating conditionwithout contacting the surfaces of the cylinder so that the piston maybe operated at high speed and without substantially any wear on themoving parts thereof.

3,285,142 Patented Nov. 15, 1966 Still a further object of the inventionis to provide a piston type cryogenic apparatus in which theconstruction and control thereof are considerably simplified over priordevices through the elimination of valve mechanisms having moving parts.

Another object of the invention is to provide apparatus of the characterdescribed in which a floating stroke of the piston is achieved by meansof gas lubrication, there being a plurality of annular grooves along thelength of the working surface of the piston or the cylinder, and thegrooves being of such axial distance apart and the clearance spacebetween the piston and wall being of such configuration as to achievethe desired floating effect, irrespective of the direction of flow ofthe gas.

Still another object of the invention is to provide a piston typecryogenic apparatus in which the valve gear is eliminated by providingsuitable inlet and outlet chambers in the cylinder itself and means inthe piston for operating the same for achieving the introduction,expansion and expulsion of the gas.

According to the invention, the piston or the cylinder of the apparatusis provided on its walls with fine annular grooves spaced along thelength of the working surface, the axial distance between adjacentgrooves being between l and the circumference of the piston, and thesurface of the piston or cylinder is so formed that the radial clearancespace varies, being different dimensions axially along the length of thepiston between any two adjacent grooves.

The invention is further characteriezd by the provision of means in thepiston itself to enable the same to function as a control valve, inletand outlet ports and chambers being formed in the bearing surface of thecylinder.

Other objects and advantages of the invention will appear from thedescription of preferred embodiments which follow in connection with thedrawings which illustrate the same. The same characters of reference areused throughout the several figures of the drawings to illustrate thesame or equivalent parts wherever feasible.

In the said drawings:

FIG. 1 is a fragmentary diagrammatic view on a greatly exaggerated scaleshowing one side of the radial clearance space between a piston andcylinder constructed in accordance with the invention.

FIG. 2 is a pressure diagram illustrating the flow conditions in theclearance space of FIG. 1, the principal points of pressure beingaligned for discussion.

FIG. 3 is a fragmentary schematic and diagrammatic I view of a pistonand cylinder constructed in accordance with the invention, specificallythe structure of FIG. 1.

FIG. 4 is a view similar to that of FIG. 3, but illustrating a modifiedform of the invention.

FIG. 5 is a pressure diagram similar to that of FIG. 2 but related inthis case to the construction of FIG. 4.

FIG. 6 is a schematic vertical sectional view through a cylinder andpiston unit of a cryogenic machine constructed in accordance with theinvention, the grooves and clearance space not being shown in this viewbecause of the practical problems of illustrating the same.

FIG. 7 is a schematic vertical sectional view through a modified form ofcylinder and piston unit.

FIG. 8 is a fragmentary schematic vertical sectional view of a modifiedform of the invention in which there i is an adjustable bottom plug inthe cylinder.

FIG. 9 is a chart illustrating the relative values of pressure andvolume in the working cycle of the apparatus of the invention.

Referring now to the drawings, in FIG. 1 a fragment is shown of a pistonand cylinder unit constructed in acco'mpressible medium in theisotherrnic case.

cordance with the invention, the view being sectional and taken at oneside of the piston. It will be appreciated that the dimensions aregreatly exaggerated to aid in the explanation, since the radial distancebetween the piston and the cylinder wall is'a few microns. The cylinderis designated 1, the piston 2, and 3 and 4 are two successiv annulargrooves. The direction of flow of the gas is designated by the arrow andthe space between the piston and inner surface of the cylinder isdesignated s.

Note that this space varies substantially uniformly, decreasing in thedirection of flow, so that there is a change in pressure along thelength ofthe piston. This arrange- ;ment of decrease in clearance spacemay be provided overall or part of the piston (or the cylinder surface).

The gas pressure p in the annular groove 3 is higher than the gaspressure p" in the annular groove 4.

For dimensions encountered, where, as stated, the di mension s is afewmicrons, as necessary in the practical devices, the flow of gasthrough the clearance space has approximatelythe same action as theviscous flow of a (Viscous in this case means that only the frictionalforces are taken finto consideration to the exclusionof forces due toinertia. Compressible and isother-mic means that the gas expands whileflowing through the clearance space but does not coolbecause of thermalcontact with the walls.) I r The mathematical treatment of this type offlow, considering the case of a constant dimension s, for a pressuredistribution from p to p" for any length 1 becomes:

which is recognized as aquadratic equation, indicated in FIG. 2 by thedashed line curve. As Equation 1 shows, the pressure distributionbetween two annular grooves in the case of a clearance space which isuniform, i.e., parallel, does not .depend upon the radial .distance ofthe space itself, so long as the latter remains so small that theconditions under which Equation 1 has been derived are met.

For the case that the clearance space s decreases in radial dimension inthe direction of flow of the gas, the calculation yields a pressuredistribution which corresponds qualitatively to the solid-line curve ofFIG. 2. This can readilybe seen if it isconsidered that the throtat thepoints of smaller clearance than larger. There is therefore a lowpressure drop in the annular groove'3,

tling of the gas and the'pressure.gradientmust be greater and withdecreasing clearance dimension toward the an nular groove 4, anincreasingly greater pressure drop, so that the solid curve must alwaysbe above the dashed line An important feature of the invention, and onewhich makes the floating action possible without contact'between thewalls of cylinder and the piston surface is that the piston is alwaysself-centering in the cylinder. moves out of perfect concentricity it isin an unstable condition, and forces produced tend to move it intoperfect centering, at which position it will reciprocate in a stablecondition. FIG. 3 shows in exaggerated scheand' throttling clearancespaces between the annular grooves whose radial dimension decreases inthe direction of gas flow. A stabilizing effect by the gas flowing inthe clearance Space is achieved to provide the self-' If it ,maticdiagram, a piston 2 constructed according to the. invention and havingthe annular grooves 3, 4, 5, 6

line curve of FIG. 2. This condition is approached closer with anincrease in the clearance dimension, On the opposite side of the piston,however, the clearance dimension is much narrower, and the relativevariation in an axial dimesion consequently is much greater.

The pressure distribution on the narrow clearance side of the pistonwill be substantially different, therefore, than on the wider clearanceside, considering of course the distribution along the axial length ofthe piston between annular grooves. This difference will increase as thepiston and cylinder approach one another on the narrow side, and thedistribution is like the solid line curve of FIG. 2. Because. of this,there is a higher mean static gas pressure on the narrow clearance sidethan there is on the wider clearance side, and consequently a resultantforce is exerted by the gas which tends to force the piston into aperfectly concentric position within the cylinder. The mean static gaspressure is expressed as follows:

The annular grooves on the piston circumference have the functionof-maintaining the same outlet and end pressure at all points of thecircumference for gas flow in the wedge shaped clearance space betweengrooves.

The foregoing consideration provides an indication of the optimum axialdistance between adjacent grooves.

Principally, the dilferent pressure distribution around thecircumference of the piston when eccentric can be equalized by the flowof gas around the piston. A sufliciently close spacing of the groovesmay result in the creation of a greater resistance to flow in theclearance space along the circumference than the flow from groove togroove. The gas under these circumstances would flow mainly in an axialdirection between the annular grooves and the tangential orcircumferential fiow would be of little substantial consequence. As anoptimum interval between annular grooves to give the best results Wehave found that this distance should be between and of the pistoncircumference. The foregoing considerations ap- 'ply strictly speakingonly to the piston when at rest in the cylinder, but since the pistonvelocities occurring in practice are much lower than the gas velocity inthe clearance space, the determination is a good approximation even fora moving piston.

Stabilization or self-centering by gas pressure occurs for theconstruction of FIG. 3 when the gas flows in the direction' indicated.Self-centering of the piston in the 50' which the clearance spacebetween a pair of adjacent cylinder can be achieved by a differentarrangement in 3 and 4 .being shown in the piston. The radial dimensions is maximum at 7 between the grooves, and minimum at the groove edges.In this arrangement, contrary to the action of the structure of FIGS. 1to 3, the

self-centering action of the piston due to gas pressure differential onopposite sides of the piston, does not depend upon the direction of flowof the gas in the clearance space.

FIG. 5 is a chart showing qualitativelythe pressure 1 distribution alongthe axial length of the clearance space for the flow in the direction ofincreasing 1, (from the bottorn to the top). The dashed line curverepresents the pressure distribution for a clearance space of uniform,that is, parallel, radial dimension. The solidline curve representsqualitatively the pressure distribution for the profile shown in FIG. 4.The explanation of this curve is as follows: At the start (bottom) thereis a great pressure drop due to the narrow clearance dimensionat thispoint, followed by a region of relatively low pressure drop because ofthe wide clearance dimension, and finally another 'high pressure drop atthe end because of the narrow radial clearance dimension. An analysis ofthis arrangement shows that with a symmetrical profile, that is withsimilar variation in clearance dimension on both sides of the line ofsymmetry S, the intersection of the sold line pressure curve with thedashed line lies likewise on S. This indicates clearly that the meanstatic pressure 5, which correspondsto the solid line curve, .is greaterthan that corresponding to the dashed line curve. Thus, on the basis ofthe explanation of FIG. 3, there will be a stabilizing or self-centeringeffect upon the piston in the clearance space due to gas pressure.

In FIGS. 6 to 8, there are illustrated diagrammatic views of piston typecryogenic machines constructed in accordance with the invention asdescribed above, but including additional novel features which result insimple and highly efiicient apparatus. There is a cylinder 10 with areciprocating piston 11 arranged to move therein and perform work duringthe expansion of the working gas. The piston is preferably coupled witha connecting rod 12 which can perform the work upon some externalinstrumentality (not shown). The cylinder and piston 11 define thechamber 13 which will be filled by the incoming gas.

The cylinder 10, according to the invention, is provided in the axialrange of the stroke of the piston, with an annular groove 14 connectedby several radial passageways 15 circumferentially spaced about thecylinder with an annular enclosed gallery 16 connected to a highpressure source of the Working gas (not illustrated) through a conduit17. Adjacent lower dead center of the piston movement an annular groove18 is formed in the piston connected by radially and downwardlyextending channels 19 spaced circumferentially about the piston to acentral axially extending bore 20. When this .groove 18 passes over theannular groove 14 in downward movement of the piston 11, the chamber 13will receive a charge of the Working gas at a pressure 12 When thepiston 11 rises, the groove 18 passes over and above the groove 14 sothat the gas is cut off from entering fiurt'her, and the charge iscomplete. The gas in the chamber 13 now expands and performs workthrough the connecting rod 12, the equivalent of which is withdrawn inthe dorm of heat from the working gas, thereby cooling the gas.

At the end of the expansion of the gas, when the piston is in thevicinity of its upper dead center, the bottom end 21 of the pistonuncovers an annular groove 22 formed in the cylinder wall, and theexpanded, cooled gas escapes through a plurality of radial passageways23 into the annular gallery 24 and thence passes by way of the conduit25 to the means for re-cycling the gas (not shown). The ratio ofincoming charge volume to terminal volume is chosen so that the pressurep in the chamber 13 is greater at the end of the expansion portion ofthe cycle than the counter pressure 17 in the exhaust gas line 25. Whenthe piston 11 descends, the outlet channel groove 22 is again closed andthe working gas in the chamber 13 is recompressed from 1 to 17 Since p,is lower than p same volume ratio exists in the recompression as inexpansion and 17 is less than the inlet pressure p so that after theinlet channel has been uncovered the chamber 13 can be charged oncemore. The pressure and volume values for this working cycle can be takenfrom the diagram represented in FIG. 9. V and V correspond to theinitial and terminal volumes.

There is always a pressure gradient between the annular grooves 14 and22 which is defined as p p and consequently gas flows constantly betweenthe piston and hearing surface of the cylinder, which eifects thesocalled gas lubrication of the moving piston and keeps it floatingwithin the cylinder out of contact with the surface thereof, this beingdue to the construction described in connection with FIG. 4.

The central axial bore 20 is advantageous in that the piston 11 isinteriorly scavenged by cold gas. This is important during the coolingperiod, since the coefficient of expansion of the piston and cylinderare relatively great in the range of room temperature, so that it isessential that they both cool simultaneously. If this were not achieved,the cylinder would cool faster than the piston and contract to seize thepiston.

In the embodiment of FIG. 7 the discharge of the gas is not controlledby the bottom end 21 of the piston but by the annular groove 18, thelatter also being used for charging the chamber 13. The expanded gas isreleased by the coincidence of the groove 18 with the annular groove 22in the upper part of the cylinder, this cycle also corresponding to thediagram of FIG. 9. Otherwise, the construction is similar to that ofFIG. 6.

Regulation of the intake and exhaust volume ratio can be effected by thearrangement shown in FIG. 8. Instead of a fixed bottom wall in thecylinder 10, according to FIG. 6, there is a piston-like plug 26 fittedinto the bottom of the cylinder, its position being adjusted by athreaded adjusting screw 27 axially engaged through an end cap 30 orextension secured to the cylinder. Preferably, a bellows 28 ensuresagainst escape of gas through the extension 30 or screw 27. The volumeof the chamber 13 can be varied continually during operation. Any gaswhich bleeds through the clearance space between the cylinder and theplug 26 is returned to the exhaust gas gallery 24 through a line 29.

In order to reduce as much as possible alternating heat exchange betweenthe working gas and the surfaces of the piston and cylinder in thevicinity of the expansion chamber, the piston bottom and cylinder bottomare preferably covered with a layer of heat insulating material.

Although primarily developed and best suited for use with cryogenicapparatus, the invention is likewise applicable to machines andapparatus where a piston is required to reciprocate in a cylinder andoil lubrication cannot be used.

What it is desired to secure by Letters Patent of the United States is:

1. In a machine which includes a piston reciprocating in a cylinder, andin which a gas under high pressure flows through the clearance spacebetween the piston and the inner surface of the cylinder; means foreifccting a selfcentering action of the piston to maintain the sameconcentric with the cylinder without contact with the cylinder wallsduring reciprocation, comprising a plurality of annular grooves formedin one of the piston and cylinder surfaces, axially spaced along thelength thereof, the intervening clearance space defined by the pistonand cylinder surfaces between adjacent grooves varying axially in radialdimension.

2. The structure as claimed in claim 1 in which the distance betweengroovesis between 1 and of the piston circumference.

3. The structure as claimed in claim 1 in which said radial dimensiondecreased axially in the direction of gas flow, at least over a part ofthe length of the piston.

4. The structure as claimed in claim 1 in which the radial dimension isgreater between grooves than adjacent to the grooves. l

5. The structure as claimed in claim 1 in which the grooves are in thepiston.

6. The structure as claimed in claim 1 in which the grooves are in thepiston, the radial dimension is greater between grooves than adjacent tothe grooves, and in which the profile of the piston surface issymmetrical about a line midway between adjacent grooves.

7. The structure as claimed in claim 2 in which the grooves are in thepiston and the radial dimension is greater between grooves than adjacentthe grooves.

8. A reciprocating piston machine having a cylinder and a piston movingrelative one another, inlet and exhaust means for admitting gas into thecylinder to expand for driving the piston in one direction, said inletand exhaust means being located along the length of stroke of the pistonto always provide a diiferential gas pressure along a portion of thecylinder occupied by the piston, a clearance space between the pistonand cylinder comprised of a plurality of annular grooves formed betweenthe piston and cylinder separated by radial dimension clearances axiallyvarying of such construction to cause a self-centering action of thepiston during reciprocation.

9. A machine as claimed in claim 8 in which the grooves are in at leasta portion of the length of the .piston, are separated by a distancewhich is substantially less than the circumference of the piston, and inwhich the radial clearance dimension between grooves varies along theaxial lengthof the piston.

10. A machine as claimed in claim 9 in which the radial clearancedimension is greater between the grooves than adjacent the grooves.

11. A machine as claimed in claim 8 in which the inlet and outlet meansinclude pasageways opening to the inside of the cylinder, a cooperatingannular groove in the piston adapted during reciprocation to cover anduncover the first mentioned passageways and passage means through theinterior of the piston from said annular groove to the piston end.

12. A machine a claimed in claim 8 in 'which there is an expansionchamber in one end of the cylinder, the inlet means is adapted to havesaid "gas enter tinder pressure-and leave in expanded condition, and inwhich the inlet and exhaust means are formed by recesses circumferentially disposed about the cylinder and opening to the bearingsurface thereof.

13. A machine as claimed in claim 12 in which the piston has a boreconnecting with an annular groove therein, and located relative to saidrecesses to admit gas during a short portion of the stroke in thevicinity of lower dead center of the piston, and release the expandedgas from the bottom of the piston when the end uncovers the exhaustmeans in the vicinity of the lower dead center of the piston.

14. A machine as claimed in claim 12 in which the expansion chamber isadjustable in volume.

15. A piston type cryogenic device which comprises a cylinder having acylindrical chamber therein, a piston arranged to reciprocate in thechamber and convert the heat from expanding gas into work while coolingthe gas, a link connected with the piston and extending out of thecylinder adapted to have a work absorbing apparatus coupled therewith, aplurality of grooves spaced along the length of at least a portion ofthe piston, the diameter of the piston being such relative to thediameter of he chamber to provide a clearance space of very smalldimensions, the piston having a profile between grooves to produce aradial dimension of the clearance space which varies in an axialdirection, and said radial dimension being substantially greater betweengrooves than adjacent at least one of the grooves, the grooves beingspaced axially by a dimension which is between and V the pistoncircumference, inlet and exhaust means for gas to be admitted into saidcylinder at high pressure and exhausted after cooling, the pistonstroke, dimensions of the chamber and the pressures being chosen so thatthere is always a diilerential pressure along the length of the pistonwhereby to cause the flow of some gas in the clearance space to make thepiston self-centering in the cylinder.

16. A piston type cryogenic device which comprises a cylinder having acylindrical chamber therein, a piston arranged to reciprocate in thechamber and convert the heat from expanding gas into work While coolingthe gas, a link connected with the piston and extending out of thecylinder adapted to have a work absorbing apparatus coupled therewith, aplurality of grooves spaced along the length of at least a portion ofthe piston, the diameter of the piston being such relative to thediameter of the chamber to provide a clearance space of very smalldimensions, the piston having a profile between grooves to produce aradial dimension of the clearance space which varies in an axialdirection, and said radial dimension being substantially greater betweengrooves than adjacent at least one of the grooves, the cylinder havingfirst passage means formed therein opening to said chamber to permit theintroduction of gas at a relatively high presure into the chamber Whilethe piston is atapproximately one end of its stroke, the expansion ofthe gas adapted to drive the piston in the opposite direction, with thepiston having valving means for cutting off the first passage meansshortly after the piston commences to move in said opposite direction,the cylinder having second passage means opening to said chamber andnormally closed off until the piston is approximately at the second endof the stroke, and the said piston valving means acting also to uncoversaid second passage means to enable the exhaust of expanded and cooledgas from the chamber, said chamber having dimensions chosen with respectto said stroke to provide gas pressure in said chamber at all timeswhereby to cause g-as always to be flowing in said clearance space, thepiston being self-centering because of said gas flow and piston profile.

17. A device as claimed in claim 16 in which said valving means of thepiston comprise an annular recess in the piston, a central bore in thepiston connecting With said recess and the piston end and said pistonend, the latter serving to uncover said second passageway.

18. A device as claimed in claim 16 in which the piston is hollowwhereby cooling gas may pass through the same during reciprocation sothat the temperatures of piston and cylinder change substantiallytogether during operation of said device.

References Cited by the Examiner UNITED STATES PATENTS 1,835,790 12/1931Legrand 13843 2,291,243 7/1942 Levy 91162 2,833,602 5/1958 Bayer 9l162 X3,032,017 5/ 1962 Poll'auf 91401 3,035,879 5/1962 Hanny 92-462 X3,058,450 10/1962 Lissau 92162 X MARTIN P. SCHWADRON, Primary Examiner.SAMUEL LEVINE, Examiner. P. T. CQBRIN, Assistant Examiner.

1. IN A MACHINE WHICH INCLUDES A PISTON RECIPROCATING IN A CYLINDER, ANDIN WHICH A GAS UNDER HIGH PRESSURE FLOWS THROUGH THE CLEARANCE SPACEBETWEEN THE PISTON AND THE INNER SURFACE OF THE CYLINDER; MEANS FOREFFECTING A SELFCENTERING ACTION OF THE PISTON TO MAINTAIN THE SAMECONCENTRIC WITH THE CYLINDER WITHOUT CONTACT WITH THE CYLINDER WALLSDURING RECIPROCATION, COMPRISING A PLURALITY OF ANNULAR GROOVES FORMEDIN ONE OF THE PISTON AND CYLINDER SURFACES, AXIALLY SPACED ALONG THELENGTH THEREOF, THE INTERVENING CLEARANCE SPACE DEFINED BY THE PISTONAND CYLINDER SURFACES BETWEEN ADJACENT GROOVES VARYING AXIALLY IN RADIALDIMENSION.